This grant funds research to study the use of Abelson murine leukemia virus (A-MuLV) to study early lymphocyte differentiation. Serologic and functional studies of a brain associated antigen have demonstrated it to be a differentiation antigen on immature T and B cells. Further studies have revealed it to be expressed on the entire erythroid lineage. Biochemical investigations strongly suggest that the antigen is a ganglioside or glycolipid. Using cell separation techniques, a polyvalent rabbit anti-mouse Ig, and a monoclonal rat anti-mouse brain, we have demonstrated that fetal liver cells expressing only mu chain express no sIg while fetal liver cells expressing mu and light chain do express sIg. In contrast, bone marrow cells synthesizing heavy and light chain may include a population which does not express sIg. This observation may indicate that B cell differentiation occurring in adult hematopoeitic tissue is distinguishable from differentiation occurring during ontogeny in fetal liver.